1. Field of the Invention
This invention is particularly directed to electromagnetic steel sheets which are suitable as materials for iron cores used in transformers or motors. It is particularly directed to electromagnetic steel sheets which have superior formability and magnetic properties, and to their production.
As measures to resolve increasing environmental problems such as the greenhouse effect caused by carbon dioxide emissions, the demand for electric cars is rising today. With the progress of cellular phones and Internet systems, electromagnetic shields have also been called for in the medical sector and the like. Specifically, as a material used for iron cores in small-scale electrical facilities and for electromagnetic shields, the demand for an intermediate grade of electromagnetic steel sheet is growing. An intermediate grade of electromagnetic steel sheet is one that has magnetic properties and production costs that are grouped between a grain-oriented electromagnetic steel sheet and a non-oriented electromagnetic steel sheet.
2. Description of the Related Art
A steel sheet used as a material for iron cores in transformers or motors is named an "electromagnetic steel sheet" after its applications. To this end, a grain-oriented electromagnetic steel sheet and a non-oriented electromagnetic steel sheet have been widely used.
The grain-oriented electromagnetic steel sheet is a silicon-containing steel sheet in which the grains of the sheet have been oriented in an orientation of (110) [001] or (100) [001] in the rolling direction. In the grain-oriented electromagnetic steel sheet, the grain orientation noted is generally attained by making use of a phenomenon termed "secondary recrystallization" during final finishing annealing. The technique of secondary recrystallization has heretofore been required to be performed by incorporating so-called inhibitor components in the steel material, by heating the resulting steel slab at a high temperature so as to bring the inhibitors into the form of solid solutes at high temperature, and subsequently by hot-rolling the steel slab to precipitate the inhibitors in a fine form.
For examples of inhibitors, Japanese Examined Patent Publication No. 40-15644 discloses using AlN and MnS, and Japanese Examined Patent Publication No. 51-13469 discloses using MnS and MnSe. These methods have now been implemented on an industrial basis. Use of CuSe and BN is disclosed in Japanese Examined Patent Publication No. 58-42244, and use of nitrides of Ti, Zr and V is disclosed in Japanese Examined Patent Publication No. 46-40855.
The above-mentioned inhibitor-related methods are capable of stably developing secondarily recrystallized grains. In these methods, however, the steel slab needs to be heated at a high temperature exceeding 1,300.degree. C., prior to hot rolling, to disperse precipitates in fine form. Such high-temperature slab heating places a heavy burden of cost on equipment, and moreover, causes a great deal of scale that occurs during hot rolling, eventually bringing about a low level of yield as well as a tedious task of equipment maintenance.
In producing a grain oriented electromagnetic steel sheet by use of inhibitors, final finishing annealing is usually carried out by means of batch annealing at a high temperature and for a long period of time. When left unremoved after completion of the final finishing annealing, inhibitor components tend to deteriorate the desired magnetic properties of the steel. To remove inhibitor components such as, for example, Al, N, Se and S from the steel, purifying annealing has to be effected, subsequent to secondary recrystallization, in a hydrogen atmosphere at 1,100.degree. C. or higher and over several hours. The high-temperature purifying annealing, however, makes the steel sheet product mechanically weak so that the resulting coil tends to buckle at its lower portion. Further, this effect is responsible for a sharp decline in yield.
To alleviate the foregoing shortcomings of batch annealing and to simplify the process steps, attempts have hitherto been made to convert batch annealing to continuous annealing. Methods intended for producing a grain oriented electromagnetic steel sheet by continuous annealing are disclosed in Japanese Examined Patent Publication Nos. 48-3929 and 62-31050, Japanese Unexamined Patent Publication No. 5-70833. Both of the conventional methods are designed to perform secondary recrystallization by the use of inhibitors such as AlN, MnS, MnSe and the like and within a short period of time. In practice, continuous annealing over a short period of time fails to remove inhibitor components, tending to leave the same in the steel sheet product. The inhibitor components, particularly Se and S that have remained in the steel, may obstruct the movement of magnetic domain walls, ultimately producing adverse effects on iron loss properties. Still another problem is that the inhibitor components are brittle elements which are therefore likely to render the steel sheet product less easy to fabricate. Thus, the magnetic properties and formability are not made feasible as desired, so long as inhibitors are used to achieve secondary recrystallization.
In Japanese Unexamined Patent Publications Nos. 64-55339, 2-57635, 7-76732 and 7-197126, there are disclosed methods which contemplate producing, without reliance on inhibitors, electromagnetic steel sheets having small grain diameters. The methods cited here are common to the fact that tertiary recrystallization is utilized in which priority is given to the growth of grains having a {110} plane by the use of surface energy as a driving force.
To ensure that the difference in surface energy will be effectively utilized is deemed to be the crux of each of those methods; however, the sheet thickness is required to be small so that the sheet surface is greatly receptive to and affected by surface energy. For example, Japanese Unexamined Patent Publication No. 64-55339 discloses a sheet thickness that is not more than 0.2 mm, and Japanese Unexamined Patent Publication No. 2-57653 discloses a sheet thickness of not more than 0.15 mm. In Japanese Unexamined Patent Publication No. 7-76732, no restriction is imposed on the sheet thickness, but Example 1 of this publication reveals that a sheet thickness of 0.3 mm renders the steel sheet less affected by surface energy, consequently deteriorating the integrity of grain orientation and reducing the magnetic flux density to an extreme extent, i.e., not more than 1.70 T in terms of the B.sub.8 value. Among the examples of the publication now discussed, the sheet thickness is limited to 0.10 mm so as to obtain good magnetic flux density. Also in Japanese Unexamined Patent Publication No. 7-197126, the sheet thickness is not restricted. However, since this publication is directed to a technique in which tertiary cold rolling is effected in a ratio of 50 to 75%, the sheet thickness is necessarily small, and in fact, is 0.10 mm as shown in the examples.
According to the known methods in which surface energy is utilized, the thickness of a steel sheet product has to be always small to attain good magnetic properties. Thus, a serious problem is that such a thin steel sheet product is not capable of overcoming poor punching capabilities; that is, the steel sheet product is difficult to use as a material for ordinary iron cores.
Meanwhile, the non-oriented electromagnetic steel sheet is a silicon-containing steel sheet in which the diameter and orientation of primarily recrystallized grains have been controlled by means of continuous annealing. This steel sheet is characterized by good electromagnetic properties irrespective of which direction has been subjected to rolling, but it has by far lower magnetic properties in the rolling direction than grain oriented electromagnetic steel sheets in common use.